1. Field of the Invention
The present invention relates to a nuclear power plant, and more particularly to a nuclear power plant suitably employing a nuclear reactor in which the core flow-rate is controlled by feed water.
2. Statement of the Related Art
At present, reduction in construction expenses is a major task in a nuclear power plant. With respect to a boiling water reactor (hereafter referred to as the BWR power plant), for instance, development is under way of a new type of power plant which is provided with internal pumps. In this new type of power plant, since a recirculation system used in an existing BWR power plant is not required, the overall size of a reactor container becomes compact.
However, it is desirable to make the reactor container even more compact. Examples in which the reactor container can be made compact are disclosed, though not directly referred to therein, in Japanese Patent Examined Publication No. 43-23117 (U.S. patent Ser. No. 497,787; application date: Oct. 19, 1965) and Japanese Patent Examined Publication No. 49-16920 (U.S. Pat. No. 3,621,926).
In a BWR power plant disclosed in Japanese Patent Examined Publication No. 43-23117, a portion of the feed water is supplied to a reactor pressure vessel via a feed water sparger, while the remaining portion of the feed water is used as the driving water for jet pumps so as to supply cooling water to the core. The feed water introduced into the feed water sparger is used for controlling the water level in the reactor pressure vessel. Furthermore, in this BWR power plant, the cooling water discharged from the feed water sparger and the cooling water separated by a steam separator above the core (the two kinds of cooling water are in a mixed state) is cooled by the feed water used as the jet pump driving water, and is subsequently sucked into jet pumps. The cooling of the jet pump suction water by this heat exchanger is aimed at preventing the occurrence of cavitation in the jet pumps due to the boiling of the cooling water in the jet pumps.
Japanese Patent Examined Publication No. 49-16920 discloses a BWR power plant which has jet pumps using as their driving water the cooling water discharged from a recirculation pipe and jet pumps using as their driving water a portion of the feed water, the remaining portion of the feed water being supplied from the feed water sparger into the reactor pressure vessel. In this publication as well, it is disclosed (column 6, lines 37-44) that the cooling water which is sucked from the feed water sparger into the jet pumps through an annular descending flow passage is cooled, as in the case of Japanese Patent Examined Publication No. 43-23117.
Each of the foregoing examples of the prior art is arranged such that a heat exchanger is installed in a reactor pressure vessel, and the cooling water sucked into the jet pumps is cooled by the jet pump driving water, i.e., part of the feed water, by using this heat exchanger. However, the temperature difference between the cooling water sucked into the jet pumps and the jet pump driving water, both being allowed to flow into the heat exchanger, in the order of 5.degree.-60.degree. C., which the flow rate at the side, of the heat exchanger, to be cooled is 10 times as much as the flow rate at cooling side thereof. For this reason, the heat exchange efficiency of the cooling water sucked into the jet pumps and the jet pump driving water is very poor, so that the temperature of the cooling water sucked into the jet pumps does not decline appreciably. Accordingly, in these conventional examples, the range which makes it possible to prevent the occurrence of cavitation in the jet pumps is narrow, so that a reactor output through the core flow-rate control cannot be altered substantially. In particular, it is difficult to effect a load following operation in the conventional examples.